1. Field of the Invention
The present invention relates to a camshaft with cams. More particularly, it relates to a process for producing a surface remelted and chilled layer camshaft having an excellent wear-resistant chill layer formed by melting a sliding cam surface with a high density energy, such as a TIG arc, a laser beam, a plasma arc, or an electron beam, and chilling the molten portion by self-cooling.
2. Description of the Related Art
In a camshaft with cams fitted into an engine for an automobile and the like, a sliding cam surface of each of the cams must have a superior wear-resistance. Accordingly, the cam is subjected to a surface remelting treatment (i.e., surface hardening treatment) in which the sliding cam surface portion is melted by a high density energy, such as a TIG arc, a laser beam, or an electron beam, and is rapidly cooled by self-cooling to form a chill hardened layer (for example, cf. Japanese Unexamined Patent Publication (Kokai) Nos. 59-23156, 60-234168, and 60-234169 filed by the present applicant). When the surface remelted chilled layer camshaft is produced by using this surface hardening treatment, as shown in FIG. 1, a TIG arc 5 is generated between a cam 2 of a camshaft 1 and a tungsten electrode 4 of a TIG torch 3 to melt a sliding cam surface, and simultaneously, the camshaft 1 is rotated around its center axis 6 in a direction 7 and is oscillated (reciprocated) in a direction 8 which is parallel to the center axis 6. The torch 3 is moved in a vertical direction 9, with a constant distance (gap) being maintained between the tungsten electrode 4 and the surface of the cam 2. Note, the torch 3 can be oscillated instead of the camshaft 1.
FIG. 2 shows a cross section of the cam 2 of the camshaft 1, with the axis (Z-axis) 11 of the torch 3 intersecting the center axis 6 of the camshaft 1. At a melting point A, a tangential line 12 of a cam profile and the horizontal line 13 form a varying angle .alpha. (referred to as a sag angle). The sag angle varies in the lower left side (FIG. 2) and in the lower right side (not shown) to the horizontal line as the border of the nose point. When the sag angle .alpha. is large, a problem arises in that a molten metal pool formed by a high density energy is caused to sag downward by the force of gravity. Generally, the sag angle .alpha. is at a maximum when an angle formed between the axis 11 of the torch 3 and a line connecting the cam nose point 14 to the center axis 6 of the camshaft 1 is from 15.degree. to 30.degree. (degrees). This maximum angle is formed at both sides of the cam nose point 14. One of these two positions will have the maximum sag angle during the melting by the TIG arc on a cam surface portion from a base circle portion 15 of the cam 2 to the cam nose point 14 (in FIG. 2). In this case, under the melting position, a chill layer was formed by melting and then rapidly cooled by self-cooling, accordingly the chill layer retains a certain heat. This heat delays the solidification of a portion of the molten pool that is sagging due to the force of gravity. An arc will generate preferentially between a hot spot which was melted and solidified and the tungsten electrode, so that an arc column shifts downward from a line connecting the electrode and the center axis of the camshaft to the chill layer previously formed. The faster the rotational speed of the camshaft, the larger the shift of the arc column. A portion of an argon gas stream enclosing the arc column shifted from the line flows downward along the cam surface. Furthermore, when the camshaft is rotated, a center portion of a molten metal pool is apt to flow in the rotation direction under the influence of the angular velocity. Therefore the above-mentioned factors increase the sagging of the molten metal pool. On the other hand, at the other position having the maximum sag angle during the melting by the TIG arc on a cam surface portion from the cam nose point 14 to the base circle portion 15, the sagging does not cause a problem. In this case, if the molten metal pool is caused to sag by the force of gravity, the sagging portion rapidly solidifies, since a portion of the cam under the melting position is still not heated and is cool. The arc column is shifted upward to the chill layer previously formed and continuing from the cam nose point 14, and a portion of the argon gas stream enclosing the shifted arc column flows upward along the cam surface. Therefore, the influences of the heat and argon gas stream explained in the former case do not occur, so that the sagging does not increase.
Where a large sagging of the molten metal pool occurs, as shown in FIG. 3 which is a partial cross-sectional view of a cam taken along the center axis 6, an irregularity occurs on a cam surface (i.e., a surface of a chill layer 21). In FIG. 3, a martensite layer 22 is formed under the chill layer 21, and a matrix structure of the cam (an as-cast structure) 23 exists under the layer 22. After the surface remelting treatment using a TIG arc, the surface remelted chilled layer camshaft is subjected to grinding treatment so as to form ground surfaces of cams having a predetermined profile. When a cam with large irregular surface is ground, at a recess 24 deeper than a grinding margin t, a portion of the skin remains. Generally the grinding margin t is a difference between the treated cam surface and the ground surface 27, e.g., about 0.5 mm. In practice, the grinding margin varies in accordance with the capability of a machine tool prior to the surface hardening treatment. Taking the variation into consideration, in order to eliminate the defect of the remaining skin portion, it is necessary to make a depth of the recess in the treated cam surface to be within 0.25 mm from the cam surface 26. In order to ensure the depth of less than 0.25 mm in the recess caused by the sagging of the molten metal pool, when the sag angle .alpha. is 33.degree., an arc current is decreased (the irradiation energy is decreased) to decrease the amount of the molten metal pool, whereby a maximum chill depth becomes from 0.8 to 1.0 mm. However, the chill layer having the maximum chill depth of that value is likely to became unstable, even though the wear resistance of the chill layer is such that it passes various durability tests using an engine. Preferably, the maximum chill depth is more than 1.0 mm, more preferably, more than 1.5 mm.
In order to ensure such a large maximum chill depth (chill layer thickness), the surface hardening treatment (remelting chilling treatment) on the cam surface must be carried out by using a predetermined energy controlled to ensure that the sagging of the molten metal pool due to the force of gravity is reduced.
A proposal has been made that a sag angle .alpha. be constantly kept at 0.degree. (zero degree), to minimize or prevent sagging of the molten metal pool due to the force of gravity. For example, according to a method for hardening a sliding cam surface disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-177926, a sliding cam surface portion including a nose portion between B to E in FIGS. 1 to 3 is always kept in a horizontal position (a sag angle .alpha. being approximately equal to zero). An apparatus for carrying out the proposed method requires a mechanism for eccentrically rotating a camshaft around a center axis of a small circle of the nose portion, and a mechanism for transferring a torch in a direction at right-angles to the center axis of the camshaft. In recent years, to prevent abnormal wear at a base circle portion of the cam, the remelting chilling treatment is applied on the whole circumferential surface of the cam. However the apparatus is not provided with a mechanism for treating a base circle portion of the cam. If the remelting chilling treatment for carried, the base circle portion is the camshaft is rotated around a center axis of a large circle of the base circle portion (the center axis corresponding to the camshaft center axis), so that the apparatus is very complicated.
Another proposal for decreasing the sagging of the molten metal pool due to the force of gravity has been made, wherein the torch is shifted in a direction opposite to the rotation direction from the vertical line passing the camshaft center axis 6, so as to form the sag angle in the lower right side only. This shifting of the torch is disclosed in Japanese Unexamined Patent Publication No. 53-94209, based on DE patent application No. 2703469.1. As shown in FIG. 1 of this Japanese Patent publication, the torch is arranged at an angle of 45.degree. from the vertical line passing the camshaft center axis in the opposite direction to the rotation direction. In this case, the sag angle is formed between the horizontal line and the tangential line of the cam profile, downward in the right side of the vertical line, as shown FIGS. 4a to 4f and 5a to 5f attached. In FIGS. 4a to 4f, the torch 3 is shifted in a direction opposite to the camshaft rotation direction and is arranged in such a manner that the axis of the torch 3 passes the camshaft center 6 and forms a constant angle of 45.degree. with the vertical line 11 (in the Z-direction). The remelting of the TIG arc is carried out, as shown in FIGS. 4a to 4f, while maintaining the formation of the sag angle within the lower right side to the horizontal line. Furthermore, in FIGS. 5a to 5f, the torch 3 is arranged in another manner such that the torch axis 3 forms an constant angle of 45.degree. with the vertical line 11 and intersects the vertical line 11 above the camshaft center 6. The remelting of the TIG arc is carried out, as shown in FIGS. 5a to 5f, while maintaining the formation of the sag angle smaller than that of FIGS. 4a to 4f within the lower right side. In these cases, however, since the variation of the sag angle is relatively large, arc generating spots also largely vary and a shield of an argon gas becomes irregular. Thus, the electrode of the torch is oxidized and must be discarded, so that the arc generating stops are continuously shifted from appointed positions. As a result, defects such as melt-down end portion defects and the defect of the remaining skin portion frequently occur during operation. Therefore, the frequency of electrode exchange increases, which involves an increase in the electrode costs, grinding of electrode, and an increase of exchange steps.